Follow path constraint location
First you need to put the curve origin at the point where you want the object animation to begin, so do it in Edit mode with some shiftS and shiftctrlaltC.
You also need to be sure, in Edit mode, that the curve normals go in the right direction. If it's not the case, press W > Switch Direction.
And at last, you have to keep in mind that the position of the object with the Follow Path constraint, relatively to the curve origin point, will be its position relatively to the center of the scene (i.e. Location). If the object is far from the scene center then, when you'll give it the constraint, it will jump as far from the curve origin as it's far from the scene center. If you press altG it will bring back the object to the 000 location, and therefore at the origin point of the curve.